Z hang et al. (page 273) report a link between the exocyst and the t-SNAREs used for vesicle–plasma membrane fusion. The link comes in the form of Sro7p and Sro77p. These are yeast homologues of lethal giant larvae (Lgl), which was already known to bind t-SNAREs.
Lgl is required for cell polarity and binds directly to t-SNAREs. Mutating Lgl induces polarity defects, despite the fact that it is not strictly polarized in most cells. Zhang et al. found that Sro7p and Sro77p interact with Exo84p, a recently identified member of the exocyst complex. A conditional mutant of Exo84p that failed to bind the Lgl proteins showed polarity and secretion defects. Overexpression of SRO7 suppressed the phenotype. Genetic analysis showed also that overexpression of SRO7 or upstream GTP-binding proteins, such as Rho and Rab, suppressed mutations in several other exocyst proteins.
The researchers conclude that Lgl proteins carry a signal from the GTP-binding proteins and the exocyst complex to the t-SNARE proteins, stimulating exocytosis. When overexpressed, the Lgl proteins up-regulate exocytosis and rescue cell polarity in exocyst mutants. For now the researchers think that this spatial rescue occurs via a tuning system involving the secretion machinery and polarity regulators, which may be localized to the plasma membrane during exocytosis. If that is true, they predict a similar system will be discovered in higher eukaryotes as the exocytosis machinery is well-conserved.